Bug#869480: Support multiple smarthosts (gmail support)

[Florian Weimer]

* Osamu Aoki:

> +Here, foo at example.org.sh should be as follows:
> +
> +    #!/bin/sh
> +    /usr/bin/ssh -p 22 \
> +    -i /etc/exim4/ssh/foo at host.example.org.key \
> +    -o "StrictHostKeyChecking no" \
> +    foo at host.example.org \
> +    /usr/bin/sendmail -bm -ti \
> +    -f foo at example.org

This looks quite dangerous to me because there is a large temptation
to pass data on the command line (mainly to fix the sender address),
and it is going to be very difficult to do this in a secure fashion,
without introducing shell command injection.  The end of the message
is also not signalled reliably to the sendmail subprocess (i.e. a
dropped SSH connection results in a truncated message and data loss).
This looks more like an application for BSMTP.